What ports do I need to leave open for kinit to work? - kerberos

Question

I'm trying to set up a client machine so that only kinit traffic works. So far, my rules look like
:INPUT DROP [0:0]
:FORWARD DROP [0:0]
:OUTPUT DROP [0:0]
-A INPUT -i eth0 -s 192.168.1.130 -p tcp -m tcp --sport kerberos -j ACCEPT
-A INPUT -i eth0 -s 192.168.1.130 -p udp -m udp --sport kerberos -j ACCEPT
-A OUTPUT -i eth0 -d 192.168.1.130 -p tcp -m tcp --dport kerberos -j ACCEPT
-A OUTPUT -i eth0 -d 192.168.1.130 -p udp -m udp --dport kerberos -j ACCEPT
With this as-is, when I run "kinit remuser", I keep getting the error
kinit: Cannot contact any KDC for realm 'EXAMPLE.COM' while getting initial credentials
If I run
iptables -P INPUT ACCEPT
then "kinit remuser" works as expected. And after that first success, if I run
iptables -P INPUT DROP
it still keeps working.
I've tried using wireshark to see what I'm initially disallowing. It seems arp related, but I can't consistently see that... and I thought iptables wasn't supposed to mess with arp. I'm not seeing anything missing with "arp -n" or "ip -s neigh".
I've got the client and server configured correctly enough to work, but I'm still too new to this stuff to know what other ports I need to allow to get this working. If anyone can give me some tips I'd appreciate it.
UPDATE
Although my adapter claimed to be in promiscuous mode, it wasn't. I ended up running wireshark on the server, and saw no other port numbers needed. To verify that, I updated the rules to include
-A INPUT -s 192.168.1.130 -j ACCEPT
and as the successful kinit wireshark capture suggested, it didn't help. It's only when I open up all INPUTs that it works.
Thanks.
Hector

Answer 1

Kerberos is generally udp by default. I'm not that familiar with IP tables, but while port number on the server is defined the port number on the client is entirely random. So any ip based filter has to allow incoming udp packets with arbitrary client port numbers.
Similarly on the outgoing side, you need to be able to send packets with arbitrary udp ports on the client side. For kinit you only need the kerberos port, but changing passwords, etc, you will also need the kadmin ports.
Having said all that and making my best guess at what the ip tables rules mean, I think that's what you've implemented. However, you also need access to DNS srv records or a working krb5.conf.
http://wiki.unixh4cks.com/index.php/Using_DNS_SRV_records_to_find_Kerberos_realm_servers
It may be that kinit starts working when you allow incoming DNS record lookups. This would also explain why it continues to work after the first connection as it likely the DNS record is cached on the system. Is the server listed in your krb5.conf in your /etc/hosts file? If not that would explain the behaviour.
Thinking about this a bit more, what exactly do you expect to do with this ticket after you get it? The whole point of kerberos is to implement security on an open network by securing the endpoints, not by implementing firewalls.

Answer 2

I got it. And perhaps this only applies to me or anyone else using a HW VPN. An ICMP type 3 packet needed to be received from the gateway for the krb5 transaction to continue. My co-workers who are testing something similar on a work network aren't seeing this issue, so we'll need to figure out what's special in my case and whether it applies to other things.
The hardest part of all this was finding a wired adapter that truly supports promiscuous mode.